1) I wouldn't say that single-elim is the most
efficient as much as that it is the easiest. Not the same
thing in my book.
2) One argument (not mine) for
keeping single-elim playoffs was that it was traditional.
This may just be an example of the social value of
heretics, where it is a useful example of drek, in much the
way that I voice College Bowl questions.
My
personal opinion: for large tournaments, I prefer the
divisional round-robin plus cross-bracketed playoff
divisions, a scheme which is somewhat dependent on getting a
mathematically-friendly field size. I find that the most fair. However,
that does not mean that I find all other alternatives,
including single-elimination, unfair. SE is fair, just not
the most fair.